APPARATUS AND PROCESS FOR ACTIVATION AND REACTIVATION OF WET OR DRY CARBON BY ELECTRICAL RESISTANCE HEATING IN THE PRESENCE OF STEAM

Abstract: An apparatus and process for the activation or reactivation of wet or dry carbon in a tubular reactor that consists of two or more sequential sections (9, 20, 26), each section containing a descending column of carbon, with the inlets (12, 21, 30) to introduce steam into any one or more of the sections, and wherein the electric current is fed into the column of carbon of each such section via a graphite block (8, 19, 27) serving as an electrode and positioned at the center of the top of the column by attachment to a steel plate that includes a flat or a V-shaped shelf and that is provided with openings (10, 23, 29) to allow the released gases and vapors to escape.

Description: APPARATUS AND PROCESS FOR ACTIVATION AND *, REACTIVATION OF WET OR DRY CARBON BY ELECTRICAL RESISTANCE HEATING IN THE PRESENCE OF STEAM This invention relates to an apparatus and process for the activation and reactivation of wet or c dry carbon or carbonaceous feedstocks by electrical resistance heating the presence of steam. It particularly relates to an improved apparatus in which both the electric current and the steam are more uniformly distributed through the carbon, and in which 0 the paths of both the electric current and the steam flow can be selected so as to achieve optimal flexibility in the operation of the apparatus. If the carbon or carbonaceous feedstock to be activated or reactivated is wet, the invention also relates to an 5 improved electrical resistance heating process in which the energy cost of drying is reduced, and the process is more efficient and faster and can accept carbonaceous feedstocks with higher moisture contents, larger ranges of particle size, and more variable particle size Q distributions as compared with previously described electrical resistance processes for the drying of carbon. Air or gas streams that are treated by passage through activated carbon often have significant moisture c contents, sometimes up to complete saturation. Carbons exposed to such streams may therefore contain various amount of water, up to or exceeding the weight of the base carbon itself. Carbon used in the water purification systems is necessarily wet. The first step Q in the process of reactivating such exhausted carbon is. 5 therefore, necessarily the drying of the carbon, since l the reactivation process occurs at temperatures substantially above the boiling point of water. The drying of wet powdered or granular solids requires the vaporization of the moisture followed by 5 its physical removal, usually by a stream of air or other gas. The process is hastened by the application of heat either to the wet material or to the gas stream that is made to flow over or through the material. When the powdered or granular material is carbon, electrical -*-0 resistance heating may be applied directly to the solid, as is well known in the prior art relating to the activation of various carbonaceous feedstocks or to the reactivation of spent activated carbons, as described, for example, in U.S. Patent No. 5,089,457 and prior 15 patents cited therein. Effective rapid drying of wet carbon or carbonaceous material requires temperatures at only about or modestly above the boiling point of water, such as temperatures in the range from 100 β C. to about 150°c, whereas the activation of carbon feedstocks or 20 the reactivation of spent activated carbons commonly requires temperatures of about 750 β C. or higher. When a moist or wet carbonaceous feedstock is activated or a wet spent activated carbon is reactivated by electrical resistance heating, the carbon is either 2 5 dried prior to the activation or reactivation step, or else some or substantially all of the moisture is removed in the process of the activation or reactivation. In the latter case, as the vaporized moisture moves toward the furnace exit, through and past 30 the section that preheats the carbon, it recondenses and 35 offers increased resistance to the movement of other gases (such as CO and H 2 ) that are produced in the activation process or that are desorbed in the reactivation of spent activated carbons and that must escape to the exit. Still another disadvantage is the uneven current distribution that results from the variation in electrical resistance in different portions of the moving column of the wet carbon or carbonaceous material. The use of electrical resistance heating for the activation and reactivation of carbon in the presence of steam has been described in U.S. Patents 5,089,457 and 5,173,921. In the processes described therein, the column of the carbon to be activated or reactivated descends by gravity through a tubular reactor, while steam is fed into the bottom of the reactor and moves upward in opposition to the descending carbon. The steam can undergo any or all of a number of possible reactions: 1. In carbon activation, the steam and carbon are converted to carbon monoxide and hydrogen, according to the equation H 2 O 0ai( a + C aoll< ι ---> CO ø a + H 2βm- 2. Some of the steam can react further with carbon monoxide in the "water gas shift reaction". H 2 0 β . + CO β «. > C0 2β . + H 2βM 3. The steam also serves to desorb other matter that is already adsorbed on spent activated carbons. 4. The steam can react with some adsorbates on spent carbons, as for example in the hydrolysis of adsorbed fatty esters. Other gases can also be used to activate carbon. A useful example is carbon dioxide, which reacts with carbon according to the following equation: C0 2øa . β + C me>2 _ ±Λ ——-> 2C0- 7mβ In carbon activation, reactions 1 and 2 consume steam, so that the activation process is slowed down in the upper portions of the tubular reactor, or else an excess of steam must be provided in the initial injection to make up for such consumption. Such excess seriously compromises the controls needed to establish proper reaction temperatures for activation. In carbon reactivation, the desorbed vapors from reactions 3 and 4 may recondense in the upper portions of the reactor, thereby offering increased resistance to gas flow ("back pressure") and unfavorably affecting the temperature and current distributions in the reactor. Also, if all the steam needed for its various functions is introduced at one point at the bottom of the reactor, then its rate of introduction must be limited so as not to reach a linear speed that would impose too great a pressure on the carbon or require too high a reactor temperature. These limitations necessarily slow down the activation or reactivation processes. Another disadvantage of rapid steam introduction is the possibility of lifting or incipient fluidizing of the carbon column, resulting in arcing of the electric current between carbon particles. Another disadvantage of rapid steam introduction into carbon particles that are heated by electrical resistance is the tendency for separation of the carbon particles at the injection points with consequent electrical arcing and burning of the steam injector tips. Still another major disadvantage is the fact that excessive steam flow beyond what is needed in a particular section of the reactor may have an adverse effect on the temperature profile in the reactor. In summary, the introduction at one point in the reactor of the total steam flow needed for all requirements in the reactor severely limits the options for operating controls in the activation or reactivation processes by electrical resistance heating. The present invention therefore concerns an improved process and apparatus for the drying of wet granular or powdered carbon by electrical resistance heating. Such improvement may be provided when the carbon contains any amount of moisture up to an including full saturation, so long as no free liquid water is present. Improvements can be obtained when the carbon is a carbonaceous feedstock, such as wood char, coconut char, or petroleum coke, for the production of activated carbon, or is an exhausted activated carbon that is to be reactivated. It is important in the process and apparatus of the present invention to provide ttiat the electric current passing through the carbon being dried is distributed uniformly therethrough and to insure that the moisture being removed from the carbon does not recondense on the carbon before it escapes. More specifically, the present invention is directed to an improved apparatus for the activation of a wet carbon feedstock or the reactivation of a wet spend carbon feedstock by electrical resistance heating in the presence of an activation gas that comprises a feed hopper from which the carbon feedstock enters by gravity into a tubular reactor of refractory material to form a descending column of carbon feedstock, wherein the top of the reactor is joined to the bottom of the feed hopper, and wherein the bottom of the reactor is joined to a valve for removal of the activated carbon product, an electrical terminal on the outside of the feed hopper and another terminal affixed at the bottom of the reactor, said terminals being connected to a source of electric current, the current passing between the terminals through the descending carbon feedstock, the improvements wherein the reactor consists of two or more sequential sections, each section containing a moving column of carbon feedstock, with inlets to introduce said activation gas into any of said sections in sequence, and wherein the electric current is fed into the moving column of carbon feedstock of each section via a graphite block serving as an electrode and positioned at the center of the top of the said column by attachment to a steel plate that includes a shelf and that is provided with openings to allow the emitted gases and vapors to escape. The present invention is further directed to an improved process for the activation of a wet carbon feedstock or the reactivation of a wet spent carbon feedstock by electrical resistance heating in the presence of an activation gas that comprises a feed hopper from which the carbon feedstock enters by gravity into a tubular reactor of refractory material to form a descending column of carbon feedstock, wherein the top of the reactor is joined to the bottom of the feed hopper, and wherein the bottom of the reactor is joined to a valve for removal of the activated carbon product, an electrical terminal on the outside of the feed hopper and another terminal affixed at the bottom of the reactor, said terminals being connected to a source of electric current, the current passing between the terminals through the descending carbon feedstock, the improvement comprising providing the reactor with two or more sequential sections, each section containing a moving column of carbon feedstock, with inlets to introduce said activation gas into any of said sections in sequence, and feeding the electric current into a moving column of carbon feedstock of each section via a graphite block serving as an electrode and positioned at the center of the top of the said column by attachment to a steel plate that includes a shelf and that is provided with openings to allow emitted gases and vapors to escape. In an alternative embodiment of the present invention, the activating gas is carbon dioxide instead of steam. In the present invention, the introduction of steam may be limited to any section of the reactor to the amount of steam needed for the processes occurring in that section, and the gaseous reaction products or the desorbed gases and vapors are allowed to escape from the section of the reactor where they are produced. In the present invention, the steam may be injected into the carbon by a means that will distribute the steam uniformly through the carbon particles and prevent electrical arcing and consequent burning at the steam injector points and in such a way to provide that the electric current is distributed uniformly through the carbon being activated or reactivated in each section of the reactor. The achievements of these and other aspects of the present invention will be apparent from the following description of the invention. Briefly, the present invention is directed to improvements in a process and apparatus wherein the apparatus comprises a feed hopper from which a descending column of the moist or wet carbon or carbonaceous material enters by gravity into a tubular drying section of refractory material or into a sequence of such sections, whose top is joined to the bottom of the feed hopper, and who bottommost section is joined to a reactor for activation of a carbonaceous feedstock or reactivation of a spent activated carbon (hereinafter, the term carbon feedstock will include spent activated carbon) , and which is provided with an electrical terminal affixed to the outside of the feed hopper and another terminal at the bottommost section, said terminals being connected to a source of electric current, the current passing between the terminals through the descending column of carbon particles. The improvements in the process and apparatus include feeding the current into the uppermost column of carbon particles through a graphite block serving as an electrode and positioned at the center of the top of said column and suspended above the top of the column by attachment to a steel plate that includes a flat or V- shaped shelf and that is provided with openings to allow the moisture to escape in the form of steam. Additional improvements include a sequence of one or more reaction sections that receive the dried carbon from the drying section or sections, and that are provided with the means to introduce steam into each of said reactor sections via an inlet through which the steam enters into a steam jacket provided with orifices that inject the steam at more than one distance from the central axis of the descending carbon column, and wherein the electric current is fed into the column of descending carbon feedstock in each such section via a graphite block serving as an electrode and positioned at the center of the top of the said column by attachment to a steel plate that includes a flat or a v-shaped shelf and that is provided with openings to allow the released gases and vapors to escape. The improvement in the process comprises introducing steam into at least one section of the reactor. Each steam jacket assembly, together with its adjacent portion of the tubular reactor, also serves as a heat exchanger, whereby the hot carbon gives up some of its heat to the incoming steam and thereby superheats it. If a plurality of such reactor sections is set up, and the number of such sections is in excess of that which is needed to activate or reactivate the carbon satisfactorily, the path of the electric current may be selected so as to pass through only the lowermost one or more of said sections as needed. The choice of whether or not to introduce steam into any given section is also optional and is independent of the selected path of the electric current. The shape of the graphite block is not critical. For example, it may be in the form of a portion of a rod or a tube of rectangular, circular, or other cross section. Its size relative to the cross- sectional area of the space it occupies should be large enough to provide sufficient surface area to distribute the current effectively to the carbon particles that flow past it, but no so large that it impedes the flow of said particles. For example, the cross-sectional area of the block projected to the horizontal plane may range from about or less of cross-sectional area of the opening in which it is placed to about 3/4 or even more of such area, if the carbon granules can flow effectively around it. The major advantages offered by this invention include the provision for the introduction of steam specifically into the section or sections where it is needed and allow the prompt escape of the various gases from the system in a single stage or a plurality of stages, with the simultaneous provision of greatly improved current distribution through the carbon via the graphite blbck or blocks, and -li¬ the provision of flexibility by the opportunity to select the desired number of steam-injection stages and the path of the electric current consistent with the requirements of the carbon to be activated or reactivated. Figure 1 shows an apparatus for use in the drying and activation or reactivation of carbon by electrical resistance heating according to the process of the present invention. Figure 2 shows a detail of the uppermost section of steel plate of Figure 1 with its graphite block and slotted openings. Figure 3 shows a detail of one of the steam jackets with its steam inlet, its upper and lower steam orifices, and drain for any steam condensate. In the preferred embodiments, the carbon feedstock is fed into one of three process sections, of which the first one or two may be drying sections, and which the bottommost one or two must be steam-injection sections, as will be made clearer by referring to the accompanying drawings in Figures 1, 2, and 3, which are partly diagrammatic and partly vertical sections of the apparatus 1. If the feedstock is dry, no drying section is needed, and all the sections may provide steam injection. If the feedstock is wet, at least one drying section is needed, the carbon feedstock 2 is loaded into a steel feed hopper 3, which is electrically isolated from its supports 4 by insulators 5. The feed hopper is attached to electrical terminal 6 which is fed by a DC or AC power supply 7. The carbon moves by gravity around and past graphite block 8 into the first section 9. The emitted steam and other gases escape through slotted openings 10 in plate 11. If section 9 is to function as a steam-injection section, then steam is injected into this section via tube 12 into steam jacket 13, which is provided with two sets of orifices, an upper set 14 and a lower set 15, shown in detail in Figure 3. Any steam condensate is drained out through tube 16. The effect of this arrangement of steam jacket with multiple orifices at different distances from the central axis of the carbon column is to distribute the steam more uniformly through the descending carbon column, to facilitate the use of low pressure steam at temperatures not far above 100°C. which is then rapidly superheated by the hot carbon. This heat exchange arrangement also modulates the temperature of the carbon and serves as an auxiliary means of controlling the carbon temperature. Also, the arrangement prevents electrical arcing and consequent burning at the steam injector points. It will be obvious that the same results of uniformity of steam distribution and prevention of burning of steam injector tips can be obtained by various arrangements of multiple steam orifices that embody the principles herein described. If this section is to function as a drying section, then no steam is to be injected into it. The temperature of the carbon in section 9 is measured by thermocouple 17. The dried or partially activated or reactivated carbon continues to flow by gravity through steel plate 18 around and past the second graphite block 19 into the second section 20, which is similarly provided with steam inlet 21, thermocouple 22, escape openings 23, and steam condensate drain 24. This section, too, can function as a drying section if no steam is introduced into inlet 21. A similar arrangement leads through plate 25 to the third section 26, similarly provided as in the first and second sections with graphite block 27, thermocouple 28, slotted openings 29, steam inlet 30, and steam condensate drain 31. It will be obvious to one skilled in the art that, by selecting the number of upper sections that are not provided with steam injection and that thereby serve as drying sections, and by selecting the number of lower sections through which electric current is passed, a great flexibility of drying and activation or reactivation processes is realized. Furthermore, if a combination of more than three drying and reaction sections is needed, the apparatus can be constructed to include any required number of such sections, as is evident from the description of this invention. The bottom support 32 of the apparatus, which is electrically isolated by insulators 33, is connected to the other terminal 34 of the electrical power supply. The discharge of the apparatus can be fed into a receiving storage container, such as a steel drum. The present invention will be made clearer by the following examples which are presented for illustration only, and are not considered as limiting the subject invention as claimed herein. EXAMPLE 1 A charge of activated carbon from petroleum coke that had been exposed to "liquefied petroleum gas" (mostly propane) and that contained 20% moisture by weight was introduced into the feed hopper 3. The temperatures in the three sections 9, 20, and 26 shown in the drawing were kept at 300 to 500°C. , while no steam was introduced into any of them. As the carbon progressed through the apparatus, it was found that the propane had been eliminated from the carbon and that the moisture content of the received product was less than 0.1% by weight. The carbon throughput was continued while the terminal 6 was moved down so that the current passed only through sections 20 and 26, still without any introduction of steam. It was now found that the moisture content of the received product was about 1.8% by weight while that propane elimination was still complete. The terminal 6 was again moved down, while the carbon throughput continued, so that the current passed only through section 26, still without any introduction of steam. It was then found that although the propane elimination was still complete, the moisture content of the received product was greater than 5% by weight. Accordingly, the terminal had to be returned to the upper contact so the current passed through all the drying sections if a very dry product was required. If a moisture content up to a limit of 2% would be acceptable, the terminal could be placed in the intermediate position so that the current passed only through drying sections 20 and 26. EXAMPLE 2 1 A charge of coconut shell char, which constitutes a feedstock for the production of activated carbon, was introduced into the feed hopper 3. The temperatures in the three reaction sections 9, 20 and 26 5 shown in the drawing were kept at 800 to 900 β C, and steam was injected into inlets 12, 21, and 30. AS the carbon progressed through the apparatus, it was found that the Iodine Number (A.S.T.M. Standard Test Method D 4607) ranged from 1000 to 1200, and that the Carbon ° Tetrachloride Activity (A.S.T.M. Standard Test Method D 3467) ranged from 60 to 65. These values are characteristic of a highly activated carbon, suitable for a wide diversity of commercial applications. The coconut char feedstock throughput was then continued 5 while the terminal 6 was moved down so that the current passed only through reaction sections 20 and 26. Steam was injected only into inlets 21 and 30. It was then found that the Iodine Number of the finished product was about 750 and the Carbon Tetrachloride Activity was 0 about 50. These values are typical of a good but not highly activated carbon, suitable for some commercial applications. Terminal 6 was again moved down, while the coconut char throughput continued, so that the current passed only through reaction section 26, and 5 steam was injected only into inlet 30. It was then found that the Iodine Number of the received product was about 450 and the Carbon Tetrachloride Activity was about 35. These values are too low to be generally acceptable for most commercial applications. Steam was 0 then injected into inlets 21 and 30 while terminal 6 was 5 left in position so that the current continued to pass only through section 26. It was then found that the Iodine Number of the received product was about 550 and the Carbon Tetrachloride Activity was about 45. These values are also too low to be generally acceptable for most commercial applications. Accordingly, the terminal 6 was returned to the upper contact so the current passed through all the reactions sections and steam was injected into all the inlets so as to yield a highly activated product. If a good but not highly activated carbon would have been acceptable, the terminal 6 could have been placed in the intermediate position so that the current passed only through reaction sections 20 and 26, with steam injections into inlets 21 and 30. EXAMPLE 3 A charge of activated carbon made from bituminous coal that had been exhausted (saturated) by exposure to gasoline vapors was introduced into the feed hopper 3. The temperatures in the three reaction sections 9, 20 and 26 shown in the drawing were kept at 800 to 900'C. and steam was injected into inlets 12, 21, and 30. As the carbon progressed through the apparatus, it was found that the Iodine Number (A.S.T.M. Standard Test Method D 4607) ranged from 1000 to 1200, and that the Carbon Tetrachloride Activity (A.S.T.M. Standard Test Method D3467) ranged from 60 to 65. These values are characteristic of a highly activated carbon, suitable for a wide diversity of commercial applications. The exhausted activated carbon feedstock throughput was not continued while the terminal 6 was moved down so that the current passed only through reaction sections 20 and 26. Steam was injected only into inlets 21 and 30. It was then found that the Iodine Number of the received product still remained in the range of 1000 to 1200 and the Carbon Tetrachloride Activity remained at about 60 to 65. Terminal 6 was again moved down, while the exhausted carbon throughput continued, so that the current passed only through reaction section 26. Steam was injected only into inlet 30. It was then found that the Iodine Number of the received product ranged from about 750 to 900 and the Carbon Tetrachloride Activity ranged from 45 to 55. These values are too low to be acceptable for most commercial applications. Steam was then injected into inlets 21 and 30 while terminal 6 was left in position so that the current continued to pass only through section 21. It was then found that the Iodine Number of the received product ranged from 900 to 1000 and the Carbon Tetrachloride Activity ranged from 55 to 60. These values are acceptable for many commercial applications. Accordingly, for such applications, the terminal 6 was left at the lower contact so the current passed only through the lowest reaction section, with steam injection into inlets 21 and 30. If a highly activated product would have been required, the terminal could have been placed in the intermediate position so that the current passed only through reaction sections 20 and 26, with steam injections into inlets 21 and 30.

Claims: WHAT IS CLAIMED IS: 1. An improved apparatus for the activation of a wet carbon feedstock or the reactivation of a wet spent carbon feedstock by electrical resistance heating in the presence of an activation gas that comprises a feed hopper from which the carbon feedstock enters by gravity into a tubular reactor of refractory material to form a descending column of carbon feedstock, wherein the top of the reactor is joined to the bottom of the feed hopper, and wherein the bottom of the reactor is joined to a valve for removal of the activated carbon product, an electrical terminal on the outside of the feed hopper and another terminal affixed at the bottom of the reactor, said terminals being connected to a source of electric current, the current passing between the terminals through the descending carbon feedstock, the improvements wherein the reactor consists of two or more sequential sections, each section containing a moving column of carbon feedstock, with inlets to introduce said activation gas into any of said sections in sequence, and wherein the electric current is fed into the moving column of carbon feedstock of each section via a graphite block serving as an electrode and positioned at the center of the top of the said column by attachment to a steel plate that includes a shelf and that is provided with openings to allow the emitted gases and vapors to escape. 2. The apparatus according to Claim 1 wherein said activation gas is steam. 3. The apparatus according to Claim 1 wherein said activation gas is carbon dioxide. 4. The apparatus according to Claims 1 or 2 wherein the steam is introduced into a steam jacket that is provided with multiple orifices so that steam is injected from the steam jacket into the reactor at different distances from the central axis of the column of carbon feedstock. 5. The apparatus according to any of Claims 1-4 wherein the number of such sections is 3 or 4. 6. The apparatus according to any of Claims 1-5 wherein, if the number of such sections is in excess of that which is needed to activate or reactivate the carbon satisfactorily, the path of the electric current is selected so as to pass through only the lowermost one of said sections. 7. The apparatus according to any of Claims 1-6 wherein, if the number of such sections is in excess of that which is needed to activate or reactivate the carbon satisfactorily, the path of the electric current is selected so as to bypass the uppermost one of said sections. 8. The apparatus according to any of Claims 1-7 wherein the shelf is flat or V-shaped. 9. An improved process for the activation of a wet carbon feedstock or the reactivation of a wet spent carbon feedstock by electrical resistance heating in the presence of an activation gas that comprises a feed hopper from which the carbon feedstock enters by gravity into a tubular reactor of refractory material to form a descending column of carbon feedstock, wherein the top of the reactor is joined to the bottom of the feed hopper, and wherein the bottom of the reactor is joined to a valve for removal of the activated carbon product, an electrical terminal on the outside of the feed hopper and another terminal affixed at the bottom of the reactor, said terminals being connected to a source of electric current, the current passing between the terminals through the descending carbon feedstock, the improvement comprising providing the reactor with two or more sequential sections, each section containing a moving column of carbon feedstock, with inlets to introduce said activation gas into any of said sections in sequence, and feeding the electric current into the moving column of carbon feedstock of each section via a graphite block serving as an electrode and positioned at the center of the top of the said column by attachment to a steel plate that includes a shelf and that is provided with openings to allow emitted gases and vapors to escape. 10. The process according to Claim 9 wherein said activation gas is steam. 11. The process according to Claim 9 wherein said activation gas is carbon dioxide. 12. The process according to Claim 9 or 10 wherein the steam is introduced into a steam jacket that is provided with multiple orifices so that steam is injected from the steam jacket into the reactor at different distances from the central axis of the column of carbon feedstock. 13. The process according to any of Claims 9- 12 wherein the number of such sections is 3 or 4. 14. The process according to any of Claims 9- 13 wherein, if the number of such sections is in excess of that which is needed to activate or reactivate the carbon satisfactorily, the path of the electric current is selected so as to pass through only the lowermost one of said sections. is. The process according to any of Claims 9- 14 wherein, if the number of such sections is in excess of that which is needed to activate or reactivate the carbon satisfactorily, the path of the electric current is selected so as to bypass the uppermost one of said sections. 16. The process according to any of Claim 9- 15 wherein the shelf is flat or V-shaped. 17. The process according to any of Claims 9- 110 and 12-16 wherein no steam is introduced into any of the sections. 18. The process according to any of Claims 9- ιo and 12-17 wherein no steam is introduced into the uppermost section. 19. The process according to any of Claims 9- 10 and 12-18 wherein steam is introduced only into the 0 lowermost section. 5 0 5 0 5

Inventors: DU PLESSIS CORNELIUS JACOBUS, DU PLESSIS, CORNELIUS, JACOBUS
Applicants: PLESSIS CORNELIUS JACOBUS DU, DU PLESSIS, CORNELIUS, JACOBUS
Application year: 1994
Main classification: B01J37
IPC codes: B01J20, B01J37, B01J38, C01B31, F27B9, F27D3